Millimeter wave (MMW) frequency bands span from approximately thirty GHz to over one THz. There are inherent advantages in sensing such frequency bands: all natural objects whose temperatures are above absolute zero emit passive MMW radiation, images produced by MMW may have a more natural appearance than from infrared (IR) waves, and MMWs are attenuated to a much lesser degree than visible or IR wavelengths in the presence of fog, cloud cover, snow, dust and rain. As used herein, IR includes radiation defined generally by wavelengths 0.7 μm<λIR<30 μm; and MMW includes radiation defined generally by wavelengths 280 μm<λMMW<1.5 cm. Energy in the MMW region is approximately 108 times less than that emitted in the longer wavelengths of the IR region, current MMW receivers substantially offset that disadvantage through larger pixel size (104) and temperature contrast (about 103 times better) as compared to typical IR sensors. The use of MMW detection has applications in aviation (airport/aircraft safety, all-weather vision), medical and plasma diagnostics, non-destructive testing for voids and delaminations within composite materials, remote sensing of agricultural and environmental conditions, and a wide variety of defense, counter-terrorism, and law enforcement applications.
In previous implementations (see for example, U.S. Pat. No. 6,329,655, herein incorporated by reference), MMW sensors use one, or more often an array of thermal detectors, specifically bolometers, that each provide a pixel input into a resulting image of the radiation sensed. Each thermal detector converts absorbed electromagnetic energy into an electrical signal. Since MMW radiation typically generates a low level signal, thermal insulation of each bolometer is critical to ensure the MMW signal is differentiable from noise. Typically, each bolometer is suspended by a microbridge in a non-contact position over a substrate on which they are mounted, and the microbridge is supported by conductive leads or legs. Sensors of this type are fabricated using fine patterning, micromachining or photolithography techniques. To maintain thermal isolation of each bolometer from one another and from the substrate, while ensuring coupling of the bolometer to the frequency band sought to be absorbed, antennas tailored for the desired frequency band are married to each bolometer. For MMW, the antenna and the bolometer may be disposed on the same side of a substrate.
There are generally two disparate architectures by which MMW sensors having both a sensing antenna and a bolometer have proceeded: capacitive coupling and resistive coupling. Capacitive coupling is a recent approach (e.g., U.S. Pat. No. 6,329,655) whereby opposed ends of the microbridge overlie opposed portions of an antenna, and are spaced therefrom by a gap, preferably 0.2–1.0 microns. Inductance across the gaps at opposed ends drives a current through the bridge on which the bolometer lies. Incident radiation causes temperature fluctuations within the bolometer, which are transferred to the bridge. These temperature changes register as changes in the resistance of the bridge, which is the measure of incident radiation (plus any system noise).
Capacitive coupling generally provides improved sensitivity over resistive coupling, but that advantage is offset by limits on miniaturization. Specifically, capacitive coupling requires a substantial overlap area between the bridge and the antenna such that coupling capacitive impedance ( 2/2πfCAoverlap) must remain small compared to the load impedance (100–300 Ohms) in order to be effective. Capacitive coupling efficiency scales positively with smaller gaps between the bridge and the antenna in the overlap areas. For example, for a gap of 0.1 microns, a load resistance of 100 Ohms and an incident radiation frequency of 100 GHz, maximum coupling efficiency is approximately 30% for a bridge spanning 50 microns, and approximately 70% for a bridge spanning 100 microns. A more conservative gap of 0.5 microns reduces the coupling efficiency to about 5% for the 50-micron bridge and to about 30% for the 100-micron bridge. These represent the coupling efficiency for the most optimum frequencies. To overcome this disadvantage, capacitively coupled sensors must ensure sufficient overlapping surface area (Aoverlap) at each opposed end of the bolometer bridge. Considering that most applications use arrays of multiple MMW sensors deployed on a single substrate, the requirement for high Aoverlap limits each sensor to a minimum size and correspondingly limits their practical utility in densely packed array formats.
Resistive coupling is the more traditional architecture for MMW sensors, wherein the bridge comprises a conductor such as NiCr (hereinafter, nichrome) that physically contacts each opposed portion of the antenna. While the effective coupling efficiency for resistively coupled sensors is approximately three times that for capacitively coupled sensors, sensitivity is reduced by a factor of ten to twenty as compared to capacitive architecture. Heat transferred through legs of the nichrome bridge is a source of electromagnetic noise to the temperature-sensitive bolometer. That noise reduces the sensitivity of the sensor. The above limitation was the motivation behind developing capacitive coupling.
What is needed in the art is a sensor for MMW detection and preferably also IR detection that maintains an efficient coupling of the thermal detector to the radiation wavelength of interest, that offers a high degree of sensitivity to resolve scene radiation while limiting noise levels, and that can be miniaturized without substantial performance penalty. An efficient method of making such a sensor, and arrays of such sensors, is also needed.